This invention relates to a device and method for holding catalyst in a radial flow reactor used as a reforming reactor in a petroleum refinery, an ammonium synthesis apparatus in an ammonium production plant or the like.
As a reforming reactor in a petroleum refinery plant or an ammonium synthesis apparatus in an ammonium production plant, a radial flow reactor is a suitable reactor for its high efficiency of contact between fluid and granular catalyst. A radial flow reactor is, as is well known, a reactor in which the materials being processed flow radially inward through a catalyst bed and into a chamber communicating with an outlet conduit and this catalyst bed is formed in a generally vertically erected cylindrical configuration. For building a radial flow reactor, therefore, it is necessary to form a vertically erected cylindrical catalyst bed. For forming such cylindrical catalyst bed, there are two conventional methods as will be described below.
One of the conventional methods of forming a cylindrical catalyst bed relies on employment of a center pipe screen a and scallop screens b as shown in FIG. 10.
As the scallop screens b, slitted plates of a relatively small thickness are used for necessity of forming a large number of slits c and for facility of processing and these screens b are formed in a scallop shape for preventing deformation or collapse. Since the scallop screens b are not strong enough to stand pressure of catalyst filled in the catalyst bed by themselves, these scallop screens b are disposed along the inner wall of a reactor d.
The center pipe screen a is erected in the center of the reactor d in a self-supporting manner. Since the center pipe screen a which is subject to a strong catalyst pressure is made of wire netting or a perforated plate and has not sufficient strength to stand the catalyst pressure, a perforated pipe e of a large thickness is provided inside of the center pipe screen a for reinforcing it. These scallop screens b and the center pipe screen a are installed independently from each other by separate installation work and upon completion of the respective screens a and b, catalyst is filled in an annulus formed between the center pipe screen a and the scallop screens b and, as a result, a vertically erected cylindrical catalyst bed is formed. In using the radial flow reactor, fluid is generally supplied from an inlet f located in the upper portion of the reactor d. The fluid then enters the cylindrical catalyst bed from the scallop screens b for a predetermined catalytic reaction and then is collected in the center pipe screen a and led to an outlet provided in the lower portion of the reactor d.
Conversely, fluid may be introduced from the center pipe screen a and collected from the scallop screen b and led to the outside of the reactor. Likewise, the inlet for fluid may be provided in the lower portion of the reactor and the outlet in the upper portion of the reactor.
The other method for forming a cylindrical catalyst bed in a radial flow reactor employs, as shown in FIG. 11, an inner cylindrical screen g and an outer cylindrical screen h.
The inner cylindrical screen g is of a similar construction to the center pipe screen a of FIG. 10 and and is installed in substantially the same way as the center pipe screen a.
The outer cylindrical screen h is, as is different from the scallop screens b of FIG. 10, installed in a self-supporting manner and, for this purpose, has a reinforced cylindrical construction. A cylindrical catalyst bed is formed by filling catalyst in an annulus formed between the outer and inner cylindrical screens h and g.
These conventional radial flow reactors have, however, several problems which have remained unsolved to date.
First, for obtaining a catalytic reaction of a high efficiency in a radial flow reactor, time of contact of fluid with catalyst, i.e., distance of passage of fluid through catalyst, needs to be uniform. For this purpose, the catalyst bed needs to have a uniform thickness in radial direction throughout its entire height, i.e., a uniform radius in all cross sections of the cylinrical configuration.
In the radial flow reactor using the scallop screens b and the center pipe screen a shown in FIG. 10, however, the outer periphery of the cylindrical catalyst bed is defined by the shape of the scallop screens b and therefore this catalyst bed cannot inherently attain a uniform thickness in radial direction.
A catalytic reaction in a radial flow reactor is generally performed under a high temperature and a high pressure. Since the scallop screens b are disposed along the inner wall of the reactor d, a gap tends to develop between adjacent scallop screens b due to thermal expansion and contraction of the scallop screens b occurring during running and stopping of the reactor. This causes a part of catalyst to enter space between the inner wall of the reactor d and the rear side of the scallop screens b through this gap formed between the adjacent scallop screens b with a resulting loss of efficiency in the catalyst reaction.
Moreover, when the center pipe screen a which is fixed on the bottom pate of the reactor d is even slightly inclined due to an installation error, the center pipe screen a cannot have a concentric relation particularly in its upper portion with respect to the inner wall of the reactor d with the result that a uniform thickness in radial direction of the cylindrical catalyst bed cannot be obtained. Thus, it is extremely difficult in the radial flow reactor using the scallop screens b and the center pipe screen a to form a cylindrical catalyst bed having a uniform thickness in radial direction.
In the radial flow reactor using the inner cylindrical screen g and the outer cylindrical screen h, the scallop screens b are not used and, therefore, the inherent lack of uniformity in thickness of the catalyst bed in the radial flow reactor using the scallop screens b as described above does not exist. Since, however, the inner cylindrical screens g and the outer cylindrical screen h are installed independently and separately from each other, there exists in this radial flow reactor the same problem as in the radial flow reactor using the scallop screens b that a slight inclination between the inner cylindrical screen g and the outer cylindrical screen h due to an installation error leads to lack of uniformity in thickness in radial direction of the cylindrical catalyst bed. Accordingly, it is also difficult to form a cylindrical catalyst bed having a uniform thickness in radial direction by this radial flow reactor.
Aside from the above described problem of difficulty in obtaining a cylindrical catalyst bed having a uniform thickness due to the shape of the screen element and inclination of the cylindrical screens caused by an installation error, these conventional radial flow reactors have the problem that, in a case where a radial flow reactor is of relatively large dimensions, it is difficult in these reactors to attain a uniform thickness in radial direction of the cylindrical catalyst bed throughout the entire height of the vertically disposed cylindrical catalyst bed, even if there is no inclination between the scallop and center pipe screens or between the inner and outer cylindrical screens. In a radial flow reactor of a large size, the amount of catalyst used in the catalyst bed is large and pressure produced by catalyst against the cylindrical screens becomes larger in the lower portion of the catalyst bed. Since the cylindrical screens are fixed to the reactor in the top and bottom portions, they have a substantially constant thickness in radial direction in the upper and lower portion of the cylindrical catalyst bed in the vicinity of the top and bottom portions of the cylindrical screens. In the middle portion of the cylindrical catalyst bed as viewed in the direction of its height, however, the cylindrical screens tend to be bent outwardly of the catalyst bed due to pressure of the catalyst with the result that the thickness in the middle portion of the cylindrical catalyst bed becomes larger than the thickness of the cylindrical catalyst bed in the vicinity of the top and bottom plates of the cylindrical screens. This is particularly the case when a wedge wire screen which is weaker in strength than a perforated plate screen is used as the cylindrical screen. It is, therefore, difficult to attain a uniform thickness in radial direction over the entire height of the cylindrical catalyst bed.
Another problem in a radial flow reactor is that, even when the thickness of the catalyst bed is uniform, fluid does not necessarily flow straightly and strictly radially in the cylindrical catalyst bed but it sometimes flows in a direction deviated from the radial direction depending upon the condition of packing of catalyst. That is, fluid tends to flow more easily to a portion of the catalyst bed in which the catalyst is less densely packed than to a portion in which the catalyst is densly packed, thus causing a deviated flow of fluid. The distance of passage of fluid through the catalyst bed therefore tends to vary depending upon the condition of packing of the catalyst in the cylindrical catalyst bed resulting in lack of uniformity in the product of the catalytic reaction.
In a radial flow reactor, it becomes necessary after running of the reactor for a certain period of time to check a screen portion of the reactor and repair it if necessary. For such checking and repair of the screen portion, all catalyst packed in the cylindrical catalyst bed must be removed out of the reactor regardless of whether the radial flow reactor is the reactor of the type shown in FIG. 10 or the one shown in FIG. 11. After checking and repairing the screen portion, the catalyst must be filled in the cylindrical catalyst bed again. Repair of the screen portion is usually made outside of the radial flow reactor. In a case of a chemical plant, such regular check of the radial flow reactor is performed by closing all plant temporarily and so only a short period of time is allowed before running of the plant is resumed. A lot of man power therefore is required for completing such check and repair in such a short closing period of the plant. Thus, removal and refilling of all catalyst packed in the cylindrical catalyst bed not only requires a tremendous labor and cost but impair expensive catalyst and deteriorate the quality of the catalyst through the removal and refilling processes thereby adversely affecting the efficiency of the catalytic reaction.
Aside from the requirement for checking all screen portion of the reactor, there is a case where not the entire screen but only a part of the screen needs to be checked or repaired. Even in this case, all catalyst must be removed for checking or repairing the part of the screen in the conventional radial flow reactors.
For the purpose of facilitating removal of catalyst from a radial flow reactor, U.S. Pat. No. 3,758,279 discloses a radial flow reactor in which a catalyst bed is composed of a plurality of concentrically positioned cartridges each of which is closed by a bottom member and an upper member and screens made of apertured plates. In this radial flow reactor, each of the cartridges is independently removable from the reactor and catalyst can be replaced in either of the cartridges as desired.
This prior art radial flow reactor using the concentric cartridge type catalyst beds does facilitate removal of catalyst from the reactor. Even in this reactor, however, one entire annular cartrdige must be taken out of the reactor and all catalyst in the annular cartridge must be removed even in a case where only a part of the screen of the cartridge needs to be checked or repaired. Moreover, the prior art cartridge type reactor does not solve in any way the above described problems of the radial flow reactor that it is difficult to attain a uniform thickness in radial direction over the entire height of the catalyst bed due to pressure of catalyst and that the distance of passage of fluid through the catalyst bed tends to vary due to deviation of the flow of fluid from the radial direction depending upon the condition of packing of the catalyst. Further, in a case where the size of the radial flow reactor is very large, the weight of the annular cartridge containing catalyst therein is huge and it requires a crane of a tremendous power and hence it is not very realistic to use the prior art cartridge type reactor when the size of the radial flow reactor is very large.
It is, therefore, an object of the invention to provide a device for holding catalyst in a radial flow reactor which is capable of forming a cylindrical catalyst bed having a uniform thickness in radial direction over the entire height of the catalyst bed.
It is another object of the invention to provide a device for holding catalyst in a radial flow reactor which is capable of preventing deviation of flow of fluid in the catalyst bed and thereby attaining uniform distance of passage of fluid through the catalyst bed.
It is another object of the invention to provide a device for holding catalyst in a radial flow reactor which enables checking or repairing of only a part of screen portion of the radial flow reactor without removing all catalyst in the entire catalyst bed.